The present invention relates generally to bonding cured polyolefins to uncured polyolefins. In particular, the present invention relates to forming a laminate by heat fusing a sheet of cured polyolefin and a heat sealable polyolefin to produce a substantially water impermeable seal.
Rubbery olefinic polymer sheet material has widespread use in industries where it is desirable to provide a moisture proof membrane. For example, such membranes are used to line water reservoirs, waste treatment tanks, sewage lagoons, irrigation canals and industrial waste pits. Another important application is in the installation of flat roofs for commercial and industrial buildings.
The most common material used as a roofing membrane is a cured polyolefin polymer referred to in the industry as EPDM rubber. EPDM rubbers are formulated from polymers of ethylene, propylene and diene monomers. EPDM is commonly compounded with various fillers, colorants, antioxidants, extenders, or cross linking agents. Other membrane materials are available and are formed from butyl rubber, polymers of ethylene and propylene monomers referred to as EP rubber, and combinations of the above. Such rubber membranes may be vulcanized by exposure to temperatures of about 160.degree. C. for about two hours. Other membranes are also available which are formed from PVC plastic, for example.
EPDM rubber roofing membrane is available in sheets formed into 2 to 6 m widths and a variety of lengths. The sheets are typically extruded to a thickness of 1.50 mm and then vulcanized according to known means. The vulcanized sheets are stored and shipped in roll form.
For many applications where rubbery membranes are used, it is necessary to connect a number of sheets together to form a substantially continuous water impermeable membrane over the surface to be protected. Forming a substantially water impermeable seam between such membranes has proven difficult.
In the case of roofs, extremely rigorous demands are placed on the membrane, particularly in the areas where the sheets are spliced together. Roof temperatures may reach the boiling point of water when exposed to the summer sun, or may sink to -30.degree. C. or below in the winter.
In most applications, the ability of the bond to protect the underlying surface is critical. For instance, with roof membranes, it is critical to form a substantially water impermeable joint which is capable of withstanding freeze and thaw cycles, and will remain intact for the entire life of the roof.
In order to protect the membrane from wind damage, the installed membrane is typically anchored to the substrate by mechanical means. One anchoring method includes covering the membrane with ballast consisting of rounded washed river rock. Another method includes providing battens anchors. Battens anchors often are positioned at intervals along a sheet, and require mechanical fasteners to pass through the membrane into the substrate. Piercing the membrane forms a path for moisture to reach the substrate. Sealing the openings created by the battens has also proven difficult and time consuming. Another anchoring method includes installing a flock backed membrane to a substrate covered with a tacky substance to anchor the flocking.
Apart from the difficulties in installing a roofing membrane, maintenance is also difficult. It is often necessary to patch areas of such continuous surfaces to stop moisture from leaking through the membrane.
Along with the problem of extreme weather conditions encountered on a roof, an additional problem has been present when using EPDM (ethylene, propylene, diene monomer) rubber for roofing applications. Although EPDM rubber is a very durable roofing material, its properties are similar to wax in that its surfaces are slick and have low surface energy. Consequently, EPDM rubber surfaces are resistant to many adhesives. Much time and effort has gone into developing adhesives which adhere properly to EPDM rubber and similar membrane materials.
Several methods are known for sealing two surfaces of cured polyolefin sheets together to form a seam between two adjacent sheets of roofing membrane. Two known seaming methods include the use of contact cement systems and the use of pressure sensitive adhesive tape systems. For example, Fieldhouse U.S. Pat. No. 4,480,012 discloses a pressure sensitive adhesive composition formulated for bonding cured EPDM sheets. The adhesive includes a neutralized sulfonated EPDM elastomeric terpolymer, an organic hydrocarbon solvent and/or an aliphatic alcohol, a para-alkylated phenol formaldehyde tackifying resin and alkylphenol or ethoxylated alkylphenol. The surfaces to be bonded are coated with mineral oil, are abraded, cleaned, coated with the described adhesive, allowed to dry, and are forced together by means of pressure.
The use of the adhesive described in the Fieldhouse U.S. Pat. No. 4,480,012 patent requires numerous preparation steps and requires the use of solvents and mineral oil to prepare and clean the surface prior to sealing. Installing such roofing joints is tedious, time consuming and costly.
The Fieldhouse U.S. Pat. No. 4,480,012 is merely one example of the numerous pressure sensitive adhesives developed to connect roofing membranes. Still other sealing systems utilize primers and paint-on contact adhesives. Each of the seaming methods described above have proven to be costly, time consuming and tedious. Many of the earlier developed pressure sensitive adhesive systems also require the use of a variety of solvents which are believed to cause environmental and health problems.
Methods of connecting membranes other than by using pressure sensitive adhesives have also been developed. For example, thermoplastics have been used as adhesives for connecting membranes. Thermoplastic adhesives do not require the use of solvent-based primers (except to clean the surfaces to be joined, if necessary) and therefore do not generate hazardous wastes as a result of installing a membrane system. Thermoplastic adhesives are also less expensive to manufacture than contact adhesives and pressure sensitive adhesives. In practice, the process of developing thermoplastic adhesives suitable for installing in the field has been difficult.
The Renstrom U.S. Pat. No. 4,767,653 and the Levens U.S. Pat. No. 4,732,635 for example describe the use of a strip of linear low density polyethylene mounted onto a release liner for applying to the margin of a sheet of EPDM roofing membrane. The polyethylene is applied by placing the strip on a cleaned surface of cured or uncured EPDM rubber, the release strip facing away from the EPDM surface. Sufficient heat and pressure are applied to melt the polyethylene and form a bond between the polyethylene and the membrane.
The most preferred method of applying the polyethylene strip in the Renstrom U.S. Pat. No. 4,767,653 and the Levens U.S. Pat. No. 4,732,635 patents is in the factory prior to vulcanizing. The method includes contacting a polyethylene surface of the release backed adhesive strip and a surface of an uncured sheet of EPDM, and subjecting the sheet to 375 kPa pressure at 150.degree. C. for several hours to vulcanize the membrane and at the same time form a bond between the polyethylene and the membrane.
To connect two membranes having factory installed polyethylene strips located along the edges of the membranes, a first membrane sheet is positioned on the surface to be protected such that the polyethylene strip faces up. A second membrane is positioned adjacent to the first membrane such that the polyethylene strip faces down and is adjacent to the upward facing polyethylene strip. The membranes are positioned such that the polyethylene strips substantially overlap. The protective release liners are removed prior to bonding. Heat is applied either to the polyethylene surfaces or to the upper surface of the second membrane at a temperature and for a time sufficient to soften the adhesive strips. The strips are then pressed together, forming a seal.
The polyethylene strips disclosed in the Levens '635 and Renstrom '653 patents are most advantageously applied in the factory under controlled conditions. Unfortunately, unless the membrane is completely bonded to the sheet of polypropylene, the precise dimensions and placement of each sheet of membrane must be determined in advance of bonding the polypropylene strips in order for the seams to be formed where necessary on the job site.
Measurement errors, errors in the placement of particular membrane sheets, and deviations in dimensions between blueprints and actual structures to be protected lead the membrane installer to prefer adhesives that can be conveniently and rapidly applied in the field. Although a membrane could be uniformly coated on one entire surface with the adhesive film according to Renstrom U.S. Pat. No. 4,767,653 and the Levens U.S. Pat. No. 4,732,635, the cost would be prohibitive.